Method for growing crystals



Dec. 28, 1965 R. T. DOLLOFF 3,226,193

METHQD FOR GROWING CRYSTALS Filed June 21. 1962 Gas 40 38 l 20 I I8INVENTOR. RICHARD T. DOLLOF F ,4 r rok/vEr United States Patent3,226,193 METHOD FOR GROWING CRYSTALS Richard T. Dolloif, Parma, Ohio,assignor to Union Carbide Corporation, a corporation of New York FiledJune 21, 1962, Ser. No. 204,116 12 (Ilaims. (Cl. 23-191) The presentapplication is a continuation-in-part of my application Serial No.792,580, filed February 11, 1959, and which is now US. Patent 3,053,639issued September 11, 1963.

This invention relates to improved methods and apparatus for growingcrystals at high temperature using the Verneuil fusion method.

The Verneuil fusion method consists in starting crystal growth on acrystal the upper part of which is kept at a high temperature suflicientto produce a thin molten layer thereon. Powdered feed material forgrowing the desired crystal is placed in a vertical container above thegrowing crystal, and is allowed to fall thereon.

In the above described method, heat is provided to the crystal by meansof radio frequency heaters, by a furnace, a flame, or an arc. Suchheating means require the use of special containers for the crystal. Inaddition, where an oxyhydrogen flame or a direct are are employed,considerable impurities may be introduced into the material beingmelted. Less objectionable heat sources such as radio frequency requirethat the sample be conductive.

With a view to overcoming the above outlined limitations of the Verneuilmethod and apparatus, the main object of the invention is to provide amethod for growing crystals characterized by intense clean heat togetherwith a controlled atmosphere.

A related object of this invention is to provide a method for reactingmaterials and growing crystals therefrom characterized by intense cleanheat together with a controlled atmosphere.

These and related objects, features and advantages of the presentinvention will be more fully understood as the description thereofproceeds, particularly when taken in conjunction with the accompanyingdrawing wherein FIG. 1 is a cross-sectional elevational view ofcrystalgrowing apparatus in accord with the invention; and FIG. 2 is aschematic view depicting an arc image furnace containing the apparatusshown in FIG. 1.

Broadly construed, the apparatus of the invention comprises atransparent enclosure having crystal support means, gas inlet and gasoutlet means, and powder feed means. A source of radiant energy isassociated by means of mirrors with the above apparatus in such a waythat energy can be reflected and refocused to an image on the crystalsupport at some distance from the source.

The method of the invention broadly comprises providing a suitablecrystal support means in an enclosed chamber and at the image spot of amirror, feeding growth material into the chamber and on the supportmeans, supplying the mirror with energy from a radiant energy source,thereby growing a crystal from the growth material, and cooling thecrystal. A seed or pseudo seed crystal is preferably placed on thesupport means in accord with the above method to promote the formationof a crystal. As used herein, the term growth material includes thosematerials which react to form a product which in turn forms the crystalas well as those materials which form a crystal directly.

Referring to the drawing, FIG. 1 shows details of a crystal growthapparatus, in accord with the invention. The apparatus describedcomprises a transparent enclosure suitably made of quartz or other likematerial, and having outlet mean-s 12 at its bottom. As will be observedon FIG. 2, enclosure 10, which can be evacuated or filled with inertgases, depends from the top 14 of a chamber 16. Enclosure 10 has aflared-out top section, which cooperates with base members 18 and 20 tosecure the enclosure in place, and in air-tight engagement with thelower extremity of crystal feed mechanism 30. Depending from member 20and extending into enclosure 10 is an adjustable crystal support 24, inthis example, a bent high melting point support having a flattened-outextremity 0r pedestal 26, which may be Water-cooled if desired. Thepedestal is so positioned with respect to concave mirror 27 surroundingenclosure 10 as to lie at its image spot.

Depending from the top of chamber 22 is a crystal feed mechanismcomprising a funnel-shaped member 28 provided with a gas-tight cover 40and gas inlet 38, and having an elongated tapered extremity 30 passingthrough member 20 and extending immediately above crystal support 26. Asuitable rapping mechanism 32 communicates with perforated shaker 34through cover in such manner that feed material contained in shaker 34is delivered at a controllable rate through the tapered tube 30 to thegrowing crystal supported by pedestal 26.

The crystal growing assembly of the invention is shown in its completeform on FIG. 2. The assembly comprises a radiation chamber 36 mounted ata suitable distance from crystal-growing chamber 16.

As shown, chamber 36 contains a concave mirror 48 at the near focus ofwhich is secured a water-cooled, high output illuminating carbon are 35.The arc image is transferred, in the manner shown by arrows in thedrawing, through openings 37 and 39 in chambers 36 and 16, respectively,to a water-cooled plane or corrected plane mirror 41 mounted at an angleof 45 in chamber 16. If desired, transparent walls will serve the samepurposes as openings 37 and 39. Concave mirror 27 is verticallypositioned with respect to mirror 41, so that in accord with well knownlaws of optics the radiant heat is concentrated on the growing crystal.Energies of 1500 watts per centimeter squared over a 1 centimetersquared area have been obtained with the described apparatus, using asingle are source and a plane deflecting mirror. Ob viously, variouscombinations of arc sources and optical systems can provide even higherenergy densities.

In the present apparatus, the particles of feed material aresymmetrically heated as they fall toward the image. Upon reaching theimage area, the particles are melted by the intense radiant energy, andfinally grow as a crystal or crystals on the seed surface. If desired, arod of the material to be melted may be substituted for the powderedform, and may be fed into the image spot at a suitable regulated rate.

Since the seed is usually of the same composition as the material beingmelted, there is no need in these cases for crucible to withstand thevery high temperature employed here. The top surface of the seed ismaintained in the molten state; therefore, the only material with whichthe growing crystal comes in contact is usually that of its owncomposition. Such a construction permits the growth of crystals eitherunder vacuum or under pressure, as desired.

The instant apparatus and method are particularly suitable for thepreparation of single crystals of materials such as molybdenum. Sincemolten molybdenum is quite reactive particularly at the temperaturesreached in crystal growing, it is most difficult to prepare pure singlecrystals of this material. Other workers have grown molybdenum crystalsby introducing the metal in some form directly into the arc stream, andthus melting it. Again this technique may result in some contaminationof the crystal and necessarily involves the use of a gaseous atmospherein the crystal-growing region.

The use of a vacuum, as is possible with the instant apparatus, isimpossible with prior art techniques.

As illustrative of the practice of the present invention, high qualitysingle crystals of molybdenum were grown. One of the crystals measuredinch in length by inch in diameter, and was of 99.9] percent purity andfree from inclusions and grain boundaries. The crystal growing operationwas carried out for periods of about five minutes at arc currentsapproaching 290 amperes in an atmosphere of argon. Feed rates were aboutmm. per minute. Initial melting began at about 220 amperes, and thecurrent was steadily increased up to 290 amperes during the remainder ofthe experiment.

The method and apparatus of the invention may also be used to preparehigh purity crystals of compounds formed by direct reaction of arefractory powder with a-selected atmosphere in the region of the image,or formed by the reaction of gaseous materials. By the use of highpurity refractory metal powders such as Ti and Zr, and a purifiedatmosphere of nitrogen, the production of single crystals of TiN and ZrNof purities one or two orders of magnitude better than those now commonin the field becomes an easy matter. Other nitrogenous atmospheres, suchas ammonia, may also be used, but the purity of the crystal may belowered somewhat due to the other constituents present. Thus, singlecrystals of AlN have been prepared by the reaction of Al powder in anitrogen atmosphere with subsequent crystal growth. These crystalsmeasured 1 mm. in length by 0.01 mm. in diameter. Their purity is high,as is indicated by their transparent nature. Doping agents may be addedto the material produced, such as AlN, to provide the energy absorptionnecessary for the growth of larger crystals, or to provide a change inthe semiconducting characteristics of the crystal. These agents may beadded to the crystal in any manner which is suitable for obtaining thedesired final product, such as in mixture with the feed material.

In addition, when suitable reactive gases are introduced into thechamber near the support means, the gases will react in the vicinity ofthe support to form a product in a powder form, and then a crystal canbe grown directly from this product. Crystal growth will proceed as ifthe powdered product had been originally introduced in solid form. Inthese cases, the product may form at temperatures below those requiredfor reasonable crystal growth rate; thus, the hot image spot shouldpreferably be maintained as the only hot area in the system. Asexamples, crystals can be grown as described above after the followingreactions;

It will be obvious to those in the art that many other reactions ofgaseous materials will proceed in a similar manner to yield a materialfrom which crystals can be grown.

Stoichiometry of the crystals of compounds formed by the method of theinvention may be closely controlled by adjustment of the composition andpressure of the atmosphere in the growth chamber, and also by adjustmentof the composition of the feed material. For example, in growing TiB asmall excess of boron may be added to the feed material to counteractthe preferential loss of boron from the melt, if stoichiometric orboronrich TiB crystals are desired. Alternatively, an excess of titaniumin the feed material would result in titanium-rich TiB crystals.

Many other materials may be melted and crystals grown therefrom by thistechnique. A few examples are silicon, titanium, vanadium, germanium,tungsten can bide, silicon carbide, titanium carbide, vanadium carbide,molybdenum carbide, zirconium diboride, titanium diboride, tungstenboride, zirconium nitride and tantalum nitride. This method may be usedto prepare crystals from materials having melting points up to about4000 C., such as those discussed earlier. Crystals from metals orcompounds melting at 1000 C. or lower can be grown if suitable controlssuch as filters or shutters are incorporated to regulate the intensity,and spectral distribution and exposure time of radiation on the sample.

What is claimed is:

1. A method for growing a crystal from materials having melting pointsup to about 4000" C., which method comprises providing crystal supportmeans in an enclosed chamber and at the image spot of a mirror,supplying said mirror with energy from a radiant energy source, therebyconcentrating .heat substantially solely to said image spot of saidmirror, feeding growth material into said chamber and on said supportmeans, thereby growing a crystal from said growth material, and coolingsaid crystal.

2. A method for growing a crystal from materials having melting pointsup to about 4000 C., which method comprises providing a crystal supportmeans in an enclosed chamber and at the image spot of a mirror, placinga crystal-growth-promoting material on said support means, supplyingsaid mirror with energy from a radiant energy source, therebyconcentrating heat substantially solely to said image spot of saidmirror, feeding growth material into said chamber and on said crystalgrowth promoting material, thereby growing a crystal from said growthmaterial, and cooling said crystal.

3. The method of claim 2 wherein the interior of said chamber ismaintained under pressure.

4. The method of claim 2 wherein the interior of said chamber ismaintained under a vacuum.

5. The method of claim 2 wherein said chamber contains an inert gas.

6. A method for reacting materials having melting points up to 4000 C.and growing a crystal therefrom, which method comprises providing acrystal support means in an enclosed chamber and at the image spot of amirror, feeding said materials into said chamber and on said support,supplying said mirror with energy from a radiant energy source, therebyconcentrating heat substantially solely to said image spot of saidmirror and thereby reacting said materials and growing a crystaltherefrom, and cooling said crystal.

7. A method for reacting materials having melting points up to about4000 C. and growing a crystal therefrom, which method comprisesproviding a crystal support means in an enclosed chamber and at theimage spot of a mirror, placing a crystal-growth-promoting material onsaid support means, feeding said materials into said chamber and on saidsupport, supplying said mirror with energy from a radiant energy source,thereby concentrating heat substantially solely to said image spot ofsaid mirror and thereby reacting said materials and growing a crystaltherefrom, and cooling said crystal.

8. A method for reacting materials having melting points up to about4000 C. and growing a crystal therefrom, which method comprises placinga crystal-growthpromoting material on a crystal support means in anenclosed chamber and at the image spot of a mirror, introducing intosaid chamber a reactive gas and another material reactive with said gas,supplying said mirror with energy from a radiant energy source, therebyconcentrating heat substantially solely to said image spot of saidmirror and thereby reacting said gas and said other material and growinga crystal therefrom, and cooling said crystal.

9. A method for producing crystalline titanium nitride, which methodcomprises placing a crystal-growth-promoting material on a crystalsupport means in an enclosed chamber and at the image spot of a mirror,introducing a nitrogenous atmosphere into said chamber, feeding powderedtitanium on the surface of said growth-promoting material, supplyingsaid mirror with energy from a radiant energy source, therebyconcentrating heat substantially solely to said image spot of saidmirror and thereby reacting said titanium with said nitrogenousatmosphere and growing a crystal therefrom, and cooling said crystal.

10. A method for producing crystalline Zirconium nitride, which methodcomprises placing a crystal-growthpromoting material on a crystalsupport means in an enclosed chamber and at the image spot of a mirror,introducing a nitrogenous atmosphere into said chamber, feeding powderedzirconium on the surface of said growth-promoting material, supplyingsaid mirror with radiant energy from a radiant energy source, therebyconcentrating heat substantially solely to said image spot of saidmirror and thereby reacting said zirconium with said nitrogenousatmosphere and growing a crystal therefrom, and cooling said crystal.

11. A method for producing crystalline aluminum nitride, which methodcomprises placing a crystal-growthpromoting material on a crystalsupport means in an enclosed chamber and at the image spot of a mirror,introducing a nitrogenous atmosphere into said chamber, feeding powderedaluminum on the surface of said growth-promoting material, supplyingsaid mirror with radiant energy from a radiant energy source, therebyconcentrating heat substantially solely to said image spot of saidmirror and thereby reacting said aluminum with said nitrogenousatmosphere and growing a crystal therefrom, and cooling said crystal.

12. The method defined in claim 11 wherein a doping agent is introducedinto said chamber during crystal growth.

References Cited by the Examiner UNITED STATES PATENTS 2,461,019 2/1949Alexander 23191 2,929,126 3/1960 Bollack et a1. 23--191 X 2,973,3492/1961 Weber et a1. 23191 X 2,993,763 7/1961 Lewis 23-2235 3,086,8564/1963 Siebertz 23301 X MAURICE A. BRINDISI, Primary Examiner.

1. A METHOD FOR GROWING A CRYSTAL FROM MATERIALS HAVING MELTING POINTSUP TO ABOUT 4000*C., WHICH METHOD COMPRISES PROVIDING CRYSTAL SUPPORTMEANS IN AN ENCLOSED CHAMBER AND AT THE IMAGE SPOT OF A MIRROR,SUPPLYING SAID MIRROR WITH ENERGY FROM A RADIANT ENERGY SOURCE, THEREBYCONCENTRATING HEAT SUBSTANTIALLY SOLELY TO SAID IMAGE SPOT OF SAIDMIRROR, FEEDING GROWTH MATERIAL INTO SAID CHAMBER AND ON SAID SUPPORTMEANS, THEREBY GROWING A CRYSTAL FROM SAID GROWTH MATERIAL, AND COOLINGSAID CRYSTAL.
 9. A METHOD FOR PRODUCING CRYSTALLINE TITANIUM NITRIDE,WHICH METHOD COMPRISES PLACING ACRYSTAL-GROWTH-PROMOTING MATERIAL ON ACRYSTAL SUPPORT MEANS IN AN ENCLOSED CHAMBER AND AT THE IMAGE SPOT OF AMIRROR, INTRODUICNG A NITROGENOUS ATMOSPHERE INTO SAID CHABMER, FEEDINGPOWDERED TITANIUM ON THE SURFACE OF SAID GROWTH-PROMOTING MATERIAL,SUPPLYING SAID MIRROR WITHENERGY FROM A RADIANT ENERGY SOURCE, THEREBYCONCENTRATING HEAT SUBSTANTIALLY SOLELY TO SAID IMAGE SPOT OF SAIDMIRROR AND THEREBY REACTING SAID TITANIUM WITH SAID NITROGENOUSATMOSPHERE AND GROWING A CRYSTAL THEREFROM, AND COOLING SAID CRYSTAL.